Ia molecules are class II major histocompatibililty gene products which are co-recognized with antigen by helper T cells and by related L3T4+ T cells. These molecules are products of immune response genes and as such determine the capacity of cells from individual animals to present specific antigens to T lymphocytes. This has suggested that the structure of the highly polymorphic Ia molecules determines which antigens may be presented and which cannot be presented. To examine this in more detail, mutant Ia molecules of the I-AKappa type have been prepared by mutagenesis and selection of cells from an antigen-presenting line. These mutants have lost the capacity to present certain antigens to T cell clones which co-recognize that antigen and wild-type I-AKappa Ia molecules. Genomic clones of the Beta chain of one mutant have been obtained and the mutation shown to consist of a single base change leading to a glutamic acid yield lysine change at position 67, in the Beta domain. Thus, a single amino acid substitution profoundly effects the capacity of a class II molecule to be co-recognized with specific antigen.